Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes

first_imgIce streams provide major drainage pathways for the Antarctic ice sheet. The stressdistribution and style of flow in such ice streams produce elastic and rheological anisotropy, whichinforms ice-flow modelling as to how ice masses respond to external changes such as global warming.Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shearwavesplitting from three-component icequake seismograms to characterize ice deformation via crystalpreferredorientation. Over 110 high-quality measurements are made on 41 events recorded at fivestations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this isthe first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitudeof the splitting ranges from 2 to 80ms and suggests a maximum of 6% shear-wave splitting. The fastshear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider threemechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdlemodel (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model).Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI componentis needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment ofcracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination ofmechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropyin an ice stream, as predicted by ice fabric models.last_img

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